1. Field of the Invention
The present invention relates to an end-modified diene copolymer and a rubber composition for tire tread containing the same. More particularly, the present invention relates to a vulcanized elastomer and a rubber composition containing the same, in which the vulcanized elastomer is obtained by modifying the active ends of a conjugate diene polymer or a copolymer comprising at least one conjugate diene compound and a vinyl-substituted aromatic compound, polymerized in the presence of an organic lithium catalyst used as a polymerization initiator.
According to a recent trend in the automobile industry, the economical aspect is worth due consideration in relation to economization of natural resources and energy. Also, there is a need for guaranteeing durability, stability and savings in fuel and various approaches have been made to meet these requirements. It is the aim of the development of automobile tires to reduce the rolling resistance directly concerned to economization in fuel consumption.
The rubber of a running tire, when regularly transformed in a relatively low frequency band, has energy dispersion to provide the rolling resistance of the automobile tire. Such a resistance can be reduced with a decrease in the hysteresis loss that may occur while running the automobile tire. The measure of the rolling resistance is given as a value of tan δ (which is a loss index of the hysteresis, i.e., dynamic loss coefficient) measured at approximately 50 to 70° C. with the frequency converted to a temperature-based function according to a temperature conversion equation, Williams-Landel-Ferry equation.
On the other hand, some approaches are made to raise the wet skid resistance in order to enhance the braking performance of tires on the surface of a wet road, which is directly concerned to the stability. Automobile tires slide on the surface of a road when the driver applies the brake, and the tread part of the tires in direct contact with the surface of the road has a lot of energy loss due to frictional resistance. The wet skid resistance, which is provided by the motion of tires in a frequency band higher than the frequency band for the rolling resistance, is measured as a value of tan δ at around 0° C.
In order to satisfy the two opposite characteristics of tires, an ingredient rubber used for automobile tires includes a butadiene rubber obtained by the emulsion polymerization method, a styrene-butadiene rubber having a high cis content, a butadiene rubber having a low cis content, a styrene-butadiene rubber polymerized using an organic lithium catalyst, a natural rubber, and an isoprene rubber having a high cis content, which are used alone or in combination with two or more different types. However, these rubbers are insufficient to simultaneously meet the two requirements, wet skid resistance and rolling resistance.
Particularly, to have a low rolling resistance, the tire should have an increased content of a butadiene rubber having a low cis content or a natural rubber, a reduced amount of a filler such as carbon black, or an incremented amount of a cross-linking agent such as sulfur, in order to have a low rolling resistance.
These approaches may lower the rolling frictional resistance of tires but rather reduces the mechanical properties.
To have a high wet skid resistance, the tire should have an increased content of a styrene-butadiene rubber having a high styrene content (for example, at least about 30 wt. %) or a butadiene rubber having a high vinyl content (for example, at least 60 wt. %), or an increased amount of a filler such as carbon black or oil. These methods, however, increase the rolling resistance.
2. Description of Related Art
Methods for improving these characteristics are disclosed in Japanese Patent Kokai (Laid-Open) Nos. 57-87407 and 58-162605, in which the living end of a styrene-butadiene copolymer having an increased vinyl content using an organic lithium initiator is coupled to a tintetrachloride(SnCl4) to improve both rolling resistance and processability mixing the rubbers for tire treads. However, the tires improved by these methods are not sufficiently superior in properties to the conventional ones.
Also, methods for reacting a halogenated silicon compound such as tetrachlorosilane with the living end of a polymer are disclosed in U.S. Pat. Nos. 3,281,383, 3,244,664 and 3,692,874 and Japanese Patent Kokai (Laid-Open) No. 54-15994. The polymers obtained by these methods are excellent in mechanical strength and abrasion resistance but have poor processability with the wet skid resistance deteriorated.
Alternatively, a multi-halogen-substituted hydrocarbon such as 1,3,5-tri(bromo methyl)benzene can be used as a coupling agent, which method is disclosed in U.S. Pat. No. 3,078,254 and disadvantageous in regard to non-uniform molecular weight distribution with a low coupling efficiency and poor performance. In another method disclosed in Korean Patent No. 90-6274, the active end of a copolymer comprising conjugate diene and styrene is modified with a carbodiimide type to provide good balance of properties such as wet skid resistance, tensile strength, bounce elasticity or exothermicity.
A similar method modifies the active end of a polymer with a multi-functional compound having a diglycidyl amino group to improve the performance of tires, which is disclosed in Korean Patent Laid-Open No. 90-8331. However, there is still a need for a rubber composition having more improved characteristics, particularly, rolling resistance, wet skid resistance and abrasion resistance with the recent growing demand from the fields of industries for all-weather tires.